Improvement of Karl Fischer Moisture Measurements During Accelerated Aging Testing of Transformer Insulating Systems

The preparation of solid electrical insulating material samples for accelerated aging testing of liquid-immersed transformer insulation systems per IEEE Std. C57.100™ requires a guaranteed low moisture content, which is critical to achieve and interpret test results accurately. Multiple test methods exist to determine the amount of water present in solid electrical insulating materials, such as: the oven drying method, solvent extraction method, and Karl Fischer titration method. Even though the test standards describe sample preparation, measurement procedures, and interpretation of results, there can be variation in some specific critical parameters of these tests. Thus, this situation requires the development of so-called best practices that should be followed to ensure more reliable and accurate measurements of moisture content for such accelerated aging tests. This article describes the various test methods for measuring moisture content and their deficiencies/benefits and focuses on the development of optimum test parameters for the oven extraction method of the Karl Fischer titration moisture test to improve the accuracy of lower moisture content measurements in particular.